1. Field of the Invention
The present invention relates to electroacoustical transducers in general, and to the fabrication and configuration of components within such a transducer, in particular.
2. Description of the Prior Art
Capacitance-type electroacoustical transducers are well known in the prior art. In such transducers, a diaphragm having an insulative layer and an electrically conductive surface has its insulative layer in contact with a grooved, irregular, electrically conductive surface of a substantially inflexible disc or backplate. The periphery of the diaphragm is maintained in a fixed position with respect to a portion of the housing of said transducer and a spring force urges said backplate into tensioning engagement with said diaphragm. The insulative layer, the electrically conductive surface of said diaphragm constituting a first electrode, and the conductive surface of said backplate constituting a second electrode, form a capacitor such that when a DC bias voltage is applied across said electrodes, irregularities in said backplate surface set up localized concentrated electric fields in said insulative layer. When an AC signal is superimposed on said DC bias, the insulative layer is stressed such that oscillatory formations develop causing an acoustical wavefront to be propagated from the diaphragm. A received acoustical wavefront impinging on the insulative layer produces a variable voltage across said capacitor electrodes in the presence of a bias.
The ability to store electrical energy in a capacitance-type transducer and subsequently propagate or transmit at least a portion of this energy in the form of an acoustical wavefront is largely determined by transducer capacitance. Being able to store more energy in a capacitance-type transducer and subsequently transmit same would either enable such energy to be transmitted further or allow smaller transducers to be employed to transmit the desired amount of energy over any given distance. The capacitance of a capacitance-type transducer has often been described by the relationship C=m(KA/d) where:
C=capacitance, in farads; PA0 m=proportionality constant; PA0 K=dielectric constant; PA0 A=effective area of capacitor plates; and PA0 d=distance between plates.
One prior art capacitance-type transducer employs an insulative polyamide film, sold by Dupont Corporation under its registered trademark KAPTON, that is positioned between an electrically conductive aluminum backplate and an extremely thin layer of gold (300A) that has been vapor deposited on said KAPTON film. Gold is employed primarily because it does not oxidize when exposed to air for extended periods of time. The gold layer and aluminum backplate correspond to the "plates" of a capacitor and the KAPTON film provides the dielectric constant and establishes the distance "d" between said "plates".
As can be seen in the above equation, distance "d" between the "plates" of a capacitance-type transducer, which is solely determined by the thickness of the above-mentioned KAPTON film, has a substantial effect on transducer capacitance. A film having a thickness of approximately 0.3 mil is the smallest thickness film commercially available that includes all of the qualities that are necessary for a suitable dielectric layer in a capacitance-type transducer which places a limitation on the magnitude of the transducer capacitance that can be achieved, at least as far as manipulating the distance between the transducer diaphragm and backplate is concerned. In addition, the housing of prior art capacitance-type transducers include a protective screen structure to prevent physical damage to the above-mentioned vapor-deposited gold diaphragm layer of said capacitance-type transducer. This protective screen increases transducer cost and attenuates the amount of acoustical energy that can effectively be transmitted by such a transducer.